Introduction


Water “age” in aquifers is a critically important hydrologic variable. The age of a molecule or particle of water from a groundwater system is defined as the time required for that water particle to travel from the point of recharge to a measurement point in the aquifer. Knowledge of groundwater age can be used to infer groundwater flow paths and recognize areas of groundwater recharge. An understanding of groundwater age can be used to reconstruct contaminant loading histories or explain trends in groundwater quality, and can contribute to the understanding of groundwater susceptibility to contamination. Estimates of groundwater age also can be used to constrain groundwater flow and transport models, rates of recharge, rates of groundwater movement, and biogeochemical reaction rates.


Although groundwater age, or time-of-travel, is an important hydrologic variable, it is not possible to sample or date individual water particles. The solution to this problem is to “date” the time-of-travel of environmental tracers that are assumed to travel conservatively with the water. Environmental tracers are widespread elements, compounds, or isotopes that can be used to estimate time-of-travel along groundwater flow paths. In this study, reference to “environmental tracers,” or use of “tracers,” to estimate groundwater times-of-travel on timescales of years to decades pertains specifically to the use of chlorofluorocarbons (CFCs) (Busenberg and Plummer, 1992; Plummer and Busenberg, 2000; International Atomic Energy Agency, 2006), sulfur hexafluoride (SF₆) (Busenberg and Plummer, 2000; International Atomic Energy Agency, 2006), and the combination of tritium and helium-3 (³H/³He) (Schlosser and others, 1989; Solomon and Cook, 2000).


CFCs are anthropogenic compounds that have been produced commercially since the 1930s, volatilize into the atmosphere, and subsequently partition into water that is in contact with the atmosphere. Concentrations of CFCs have varied over time, facilitating their use in age dating. In this report, three CFCs were used for age-dating purposes: CFC-11 (CFCl₃), CFC-12 (CF₂Cl₂), and CFC-113 (C₂F₃Cl₃). SF₆ is a compound that has natural and anthropogenic sources. Like CFCs, SF₆ volatilizes into the atmosphere, and subsequently partitions into water that is in contact with the atmosphere. SF₆ concentrations have been increasing over time, facilitating the use of SF₆ in age-dating applications where natural sources of SF₆ are absent or negligible. ³H/³He refers to the combined use of ³H (tritium) and its decay product, ³He (helium-3). Radioactive decay of ³H to ³He allows elapsed time to be calculated.


Age interpretation in groundwater systems based on tracers, such as CFCs and SF_6, is complex because it is not known if the tracers travel perfectly with the water. Further, water particles and solutes (including tracers) do not all travel with the same velocity or along identical flow paths. Thus, a water sample collected at a well is composed of a mixture of water particles and solutes associated with a range (distribution) of ages. This mixture is known as the age distribution of the sample. To a first approximation, samples collected from narrow-screened wells more closely approximate a narrow age distribution than those collected from large open intervals of the aquifer. One of the simplest ways to interpret water time-of-travel from tracer data in groundwater is to assume that all solutes arriving at a sampling location, such as a well, follow the same path similar to flow through a pipe, which is commonly referred to as piston flow. For the purposes of this compilation, all ages will be interpreted using the assumption of “piston-flow” conditions. The reported “tracer-based piston-flow ages” are a means of reporting the measured data and do not represent the distribution of groundwater ages. Tracer-based piston-flow ages should be regarded as a beginning rather than an end in estimation of groundwater age in the results presented herein.


The National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey (USGS) is tasked with (1) describing the status of and trends in water quality of large, representative portions of the Nation’s water resources, and (2) providing an understanding of natural and anthropogenic factors affecting the quality of these resources (Gilliom and others, 1995). Understanding derived from analysis of tracers is an essential component of efforts to achieve these goals.


The NAWQA Program is composed of geographically and hydrologically distinct Study Units. Descriptions of Study Units and bibliographies of Study-Unit publications are available at: http://water.usgs.gov/nawqa/studies/study_units_listing.html.


NAWQA Study Units are shown in figure 1 and are listed in table A1 (appendix A). The 48 Study Units shown in figure 1 include 47 Study Units defined primarily by river basin boundaries, plus a pilot regional groundwater study, the High Plains Regional Groundwater Study. (Note that the number of and the naming convention for Study Units shown in figure 1 reflect the fact that four pairs of Study Units from the first decadal cycle of NAWQA subsequently were merged into four larger Study Units.) 


Groundwater assessments within Study Units include networks (groups related by commonality of targeted resource) of 20–30 randomly distributed wells. Major-Aquifer Studies (MASs) (synonymous with “Study-Unit Surveys”) are composed of networks of randomly distributed wells (usually domestic supply wells) in principal aquifers. Data from MAS networks contribute to the characterization of water-quality conditions in parts of principal aquifers commonly used for supply and to the characterization of how these water-quality conditions vary across the United States. Land-Use Studies (LUSs) are composed of networks of shallow wells (usually monitoring wells) in recharge areas under dominant land-use settings, such as residential/commercial land use and various classes of agricultural land use. LUS wells are sampled to provide spatial and temporal characterization of groundwater associated with various land uses, and thus contribute to the understanding of relations between groundwater quality and land use and other human as well as natural factors. One goal of LUS network design is to provide access to groundwater younger than 10 years old (Gilliom and others, 1995). Samples from MAS wells generally represent groundwater that is deeper and older than groundwater from LUS wells. Thus, MAS and LUS networks represent complementary well networks and data, and MAS and LUS networks are the core monitoring component for groundwater in the NAWQA Program. Other standard networks in the NAWQA Program include Reference (REF) wells (wells representing groundwater minimally affected by anthropogenic activities), Flow System Studies (FSSs) (assessments of processes and trends along groundwater flow paths), and wells associated with various process-based topical studies. REF wells provide baseline characterization of natural or near-natural groundwater conditions, and as such, REF well data augment other basic monitoring data. In contrast to MAS, LUS, and REF components, FSSs (Gilliom and others, 1995) and topical studies (http://water.usgs.gov/nawqa/studies/topical_studies.html) are focused, mechanistic studies that provide links between understanding derived from detailed, local-scale investigations and monitoring assessments composed of larger-spatial-scale MAS, LUS, and REF wells.


Purpose and Scope


This report summarizes tracer data that were collected from NAWQA LUS, MAS, and REF wells during the first full decadal cycle of the NAWQA Program (Federal fiscal years 1992–2001) and the first 4 years of the second decadal cycle of the NAWQA Program (that is Federal fiscal years 2002–05). [The Federal fiscal year (FY) begins October 1 and ends September 30 of the year with which it is numbered.] Samples from “OTHER” (miscellaneous) wells also are summarized for sites that were considered potentially useful to the NAWQA Program but are not components of NAWQA FSSs or topical studies. Finally, tracer data collected from LUS and REF wells in two Study Units during FY 2006–07 also are included: wells in the Puget Sound Basin (PUGT) Study Unit (tracer data from FY 2006–07 were considered more reliable than earlier tracer data) and wells in the White, Great, and Little Miami River Basins (WHMI) Study Unit (tracer data from FY 2007 were included to fill a perceived data gap). A total of 1,399 sites are included in this report: 859 LUS, 384 MAS, 21 REF, and 135 OTHER wells.


Tracer data collected from wells that were part of NAWQA FSS and topical studies were not compiled for this report because those data are being or have been assembled and interpreted separately. Tracer data from LUS, MAS, and REF wells collected after FY 2005 were not included in this report (except as noted above), but most of these post-FY 2005 tracer data are being compiled separately. Only samples from wells were assembled for this report; samples from springs were not included because of an almost exclusive programmatic focus on wells for groundwater sampling (Gilliom and others, 1995).


The tracer data of interest were the commonly collected age-dating tracers of recent recharge (tracer-based piston-flow ages on the order of decades): CFCs (CFC-11, CFC-12, and CFC-113), SF₆, and ³H/³He. Other age-dating tracers (for example, ¹⁴C) were collected infrequently and they were not compiled herein. In addition to tracer data, major dissolved gas (N₂, O₂, Ar, CH₄, and CO₂) data were assembled because they can be used to constrain the interpretation of tracer data.


The tracer data compiled in this report represent all known CFC, SF₆, and ³H/³He data associated with the targeted (LUS, MAS, REF, and OTHER) wells and collected during the targeted timeframe (FY 1992–FY 2005) from NAWQA Study Units. The 48 Study Units shown in figure 1 and listed in table A1 (appendix A) comprise the geographic extent of the search effort. Tracer data were available from 43 of the 48 Study Units.


Tracer data that previously had been interpreted and published were compiled but not reinterpreted; this honored the local understanding that went into these interpretations. For tracer data that required interpretation, the processes used are documented in this report. The previously interpreted tracer data and the newly interpreted tracer data are presented herein as estimates of tracer-based piston-flow ages. 


Tracer-based piston-flow ages based on measured concentrations of tracers in groundwater assume advective flow without mixing or dispersion. Only tracer-based piston-flow ages are presented because they are a consistent means of reporting the analytical data. In some physical settings (for example, shallow, short-screened wells in recharge areas), the tracer-based piston-flow age may reasonably represent the time-of-travel of water from points of recharge to the well. Generally, selection of the appropriate mixing model to apply to age interpretation, and evaluation of the age distribution in a water sample, depends on detailed knowledge of the hydrogeologic environment and availability of multiple tracers. Such an effort in age interpretation is beyond the scope of this report. Therefore, as a means of reporting the analytical data, and of providing an estimate of the age of each sample, this report presents only the derived tracer-based piston-flow ages for datable samples (for example, those not affected by microbial degradation, substantial mixing, or environmental contamination).


In the following section, the tracer data sets that were assembled for this report are described. This is followed by an explanation of the methods used to interpret tracer data. Limitations of the use of tracers for the purposes of estimating groundwater age are discussed; some notable cautions that pertain to these data sets are presented. Next, tracer-based piston-flow ages are provided. Finally, some insights on recharge temperatures that resulted from this large and geographically diverse data set are discussed briefly. 


First posted January 27, 2011